docs-desktop/dotnet-desktop-guide/framework/winforms/advanced/how-to-determine-the-active-mdi-child.md

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title	ms.date	dev_langs	helpviewer_keywords	ms.assetid
How to: Determine the Active MDI Child	03/30/2017	csharp vb	Clipboard [Windows Forms], copying data to MDI [Windows Forms], child windows child forms MDI [Windows Forms], activating forms MDI [Windows Forms], locating focus	33880ec3-0207-4c2b-a616-ff140443cc0f

How to: Determine the Active MDI Child

On occasion, you will want to provide a command that operates on the control that has focus on the currently active child form. For example, suppose you want to copy selected text from the child form's text box to the Clipboard. You would create a procedure that copies selected text to the Clipboard using the xref:System.Windows.Forms.Control.Click event of the Copy menu item on the standard Edit menu.

Because an MDI application can have many instances of the same child form, the procedure needs to know which form to use. To specify the correct form, use the xref:System.Windows.Forms.Form.ActiveMdiChild%2A property, which returns the child form that has the focus or that was most recently active.

When you have several controls on a form, you also need to specify which control is active. Like the xref:System.Windows.Forms.Form.ActiveMdiChild%2A property, the xref:System.Windows.Forms.ContainerControl.ActiveControl%2A property returns the control with the focus on the active child form. The procedure below illustrates a copy procedure that can be called from a child form menu, a menu on the MDI form, or a toolbar button.

To determine the active MDI child (to copy its text to the Clipboard)

1. Within a method, copy the text of the active control of the active child form to the Clipboard.

   Note

   This example assumes there is an MDI parent form (Form1) that has one or more MDI child windows containing a xref:System.Windows.Forms.RichTextBox control. For more information, see Creating MDI Parent Forms.

   Public Sub mniCopy_Click(ByVal sender As Object, _  
      ByVal e As System.EventArgs) Handles mniCopy.Click  
   
      ' Determine the active child form.  
      Dim activeChild As Form = Me.ActiveMDIChild  
   
      ' If there is an active child form, find the active control, which  
      ' in this example should be a RichTextBox.  
      If (Not activeChild Is Nothing) Then  
         Dim theBox As RichTextBox = _  
           TryCast(activeChild.ActiveControl, RichTextBox)  
   
         If (Not theBox Is Nothing) Then  
            'Put selected text on Clipboard.  
            Clipboard.SetDataObject(theBox.SelectedText)  
         Else  
            MessageBox.Show("You need to select a RichTextBox.")  
         End If  
      End If  
   End Sub  
   

   protected void mniCopy_Click (object sender, System.EventArgs e)  
   {  
      // Determine the active child form.  
      Form activeChild = this.ActiveMdiChild;  
   
      // If there is an active child form, find the active control, which  
      // in this example should be a RichTextBox.  
      if (activeChild != null)  
      {
         try  
         {  
            RichTextBox theBox = (RichTextBox)activeChild.ActiveControl;  
            if (theBox != null)  
            {  
               // Put the selected text on the Clipboard.  
               Clipboard.SetDataObject(theBox.SelectedText);  
   
            }  
         }  
         catch  
         {  
            MessageBox.Show("You need to select a RichTextBox.");  
         }  
      }  
   }  
   

See also

• Multiple-Document Interface (MDI) Applications
• How to: Create MDI Parent Forms
• How to: Create MDI Child Forms
• How to: Send Data to the Active MDI Child
• How to: Arrange MDI Child Forms